Digital VTR's can be expected to receive digital video data in a compressed format. Several formats have been proposed for compressing video data to form a digital video data stream which may then be displayed and/or recorded on video tape. For a discussion of several proposed digital video standards, see U.S. patent application Ser. No. 08/003,887 referred to above.
One digital video compression and data transmission format that offers particular promise with regard to high definition television ("HDTV") is the ISO-MPEG (International Standards Organization--Moving Picture Experts Group) standard described in a report titled "Coding of Moving Pictures and Associated Audio for Digital Storage Media up to about 1.5 Mbits/s", ISO 2 11172 rev 1, Jun. 10, 1992 hereby expressly incorporated by reference.
Terms used in this application are intended to be used in a manner that is consistent with the same terms used in the MPEG standard unless indicated otherwise. Thus, references to video pictures, I-pictures, P-pictures, B-pictures, video codewords, video codeword headers, slices, slice headers, macroblocks, macroblock headers, DCT (discrete cosine transform) coefficients and other terms used to refer to video data stream elements and compression techniques are intended to refer to such elements and techniques as defined by the MPEG standard. The use of such MPEG terminology is, however, in no way intended to limit the present application to the MPEG video data standard. Accordingly, references to MPEG data stream elements are intended to cover similar "MPEG like" data stream elements incorporated into video standards which use the same basic formats and data compression techniques described in the above referenced MPEG documents.
It is to be understood that various features of the present invention, such as data recording techniques, as opposed to, e.g., specific data prioritization and selection techniques, are generally not data format dependent and are therefore not limited to applications involving specific data formats.
While digital VTRs may have to be designed to work with one or more video compression schemes and/or data formats, the basic problems associated with increasing the recording time of a digital VTR are generally the same regardless of the format of the compressed digital video data being supplied to a VTR for recording and later playback.
Herein references to normal play modes of VTR operation are intended to refer to modes of digital VTR operation wherein data sufficient to reproduce a complete or almost complete set of the video pictures received by the VTR are recorded on, and/or read from, the video tape. Normal play modes of VTR operation are to be contrasted with trick play modes of VTR operation such as fast forward and reverse operation where only a small portion of the video data received and recorded on a tape are read and displayed during such trick play operation.
Analog video tape recorders that are capable of supporting multiple normal play modes of VTR operation are well known. For example, VHS VCR's generally support long play "LP" mode, and extended play "EP" mode in addition to the standard play "SP" mode of operation. In each of the three modes of operation, the same fixed length of tape is used to store a different number of video images, e.g., a sufficient number of images to display 2, 4, or 6 hours of National Television Systems Committee ("NTSC") analog video data. Each of these three different normal play modes of operation provide differing image quality.
The different normal play modes supported by VHS VCRs are achieved by using different tape speeds for each normal play mode of VHS VCR operation resulting in different data densities on the tape for each normal play mode. This permits the video data rate and the tape output data rate to remain unchanged in the different modes of operation. While the video data rate remains the same for all modes of VHS operation, as the tape data density is increased to support the longer play modes of operation, the signal to noise ratio ("S/N") is decreased resulting in a corresponding decrease in image quality during VHS playback operation.
Digital video tape recorders, including those that might be used to record HDTV will generally be required to operate in the highest tape data density mode possible in order to store the large amounts of digital data needed to represent video images. Thus, any attempt to increase data density on a digital video tape beyond the normal data density will result in an unacceptable digital error rate. Such a high error rate is due to the decrease in the S/N ratio which results from the use of the higher than normal tape data density. In a digital VTR the increased digital error rate that results from the use of higher than normal tape data density rates, is likely to lead to a catastrophic loss of picture. Accordingly, varying the tape data density in a digital VTR does not provide a viable means of supporting multiple normal play recording and video tape recorder playback speeds, i.e., modes of digital VTR operation, as it does in analog VCRs.
The use of data reduction techniques to reduce the amount of data required to represent a series of images might appear to be the only thing necessary for increasing digital VTR recording time. However, the mere reduction, e.g., through the use of data compression or other techniques, in the data rate required to create a series of video images, in and of itself, is insufficient to achieve a long-play mode of operation in a digital VTR. Generally, because of the difficulty of manufacturing a headwheel assembly that can be used to record video data at more than one rotational speed, known digital VTR's only support the recording of a data bit stream at a single constant data density. Because HDTV and other video formats require that a fixed number of video images be displayed during a time period of a predetermined duration, a reduction in the data rate requires that less data be recorded and later read back per a given unit of time than would be required if the data were recorded and read back at the full bit stream data rate. Thus, a digital VTR which implements a long-play mode through a reduction in the data rate is required to implement one bit stream recording and playback data rate for standard play operation and another bit stream recording and playback data rate for long-play mode operation.
Known digital VTRs are capable of recording data comprising bit streams only at a single constant data rate. Accordingly, because known digital VTR's are incapable of recording multiple-speed bit streams at a constant data density, which would be required to support a standard play and a long play mode of operation in a digital VTR, known VTRs can not support a long play mode of operation implemented using data reduction techniques alone.
While some known data logging devices based on linear scan, as opposed to helical scan, recording methods support the recording of multiple-speed bit streams, such data logging devices are impractical for use as digital VTRs. This is because linear scan data recording devices capable of high data rates generally use a large number of heads which make such recording devices too costly for use as consumer digital VTRs.
Accordingly, there is a need for a digital VTR that can support at least one long play mode of operation in addition to standard play operation. In addition, in order to maintain compatibility with standard HDTV receivers during long play mode operation, the digital VTR should generate a data stream that is compliant with the video data compression standards and data stream format used during standard play mode. Furthermore, it is highly desirable that the digital VTR be capable of being implemented in a manner that makes it practical as a consumer digital VTR.
The use of digital SDTV which will have approximately the same resolution as current NTSC television has been suggested as a method of digital television communication in addition to the various proposed digital HDTV schemes.
In the case of digital SDTV, the SDTV signal can be broadcast using a fraction of the bandwidth used to transmit a HDTV signal. Accordingly, it is possible to transmit multiple, e.g., four, SDTV signals in the same channel that could otherwise accommodate a single HDTV signal. By dividing the available TV signal spectrum into a plurality of channels having a bandwidth equal to the bandwidth of a HDTV signal, it will be possible for a broadcaster to broadcast either a single HDTV signal or multiple digital SDTV signals within a single channel. For example, a HDTV signal may be broadcast at night while multiple SDTV signals are broadcast in the same channel at other times of the day.
While most proposals for HDTV signals have suggested use of a fixed rate HDTV signal for broadcast purposes, e.g., having a bandwidth corresponding to the maximum bandwidth permitted by the size of allocated channels, it has been suggested that SDTV signals be allowed to vary in terms of their data rate. In such a case, a transmitter allocated a single channel in which multiple SDTV signals can be broadcast would be able to instantaneously vary the bit rate for any one of the multiple SDTV signals being broadcast. In this manner, a transmitter may provide for an increased bit rate for one of the SDTV signals being broadcast within a channel at the expense of the other SDTV signals being broadcast within the same channel.
In such a case, the maximum bit rate possible for any SDTV signal would be the full channel bandwidth but since the use of the full bandwidth would result in the exclusion of other SDTV signals from the channel bandwidth it is likely that a maximum permissible data rate will be set either by the industry or the Federal Communications Commission for the transmission of SDTV signals. One likely maximum data rate is the data rate specified by the MPEG main profile at main level description which specifies a maximum data rate of 15 MBits/s which is less than the expected 19.3 Mbits/s data payload of a HDTV broadcast channel. Such a maximum data rate limitation will also serve to place reasonable limits on the size of the buffers that will be required to implement a SDTV receiver.
Because digital video tape recorders are generally designed to record at fixed rates there is a need for a method and apparatus for implementing a digital video tape recorder capable of recording and reproducing SDTV video signals which may have variable transmission data rates.
In addition, it is highly desirable that a VTR be capable of recording both HDTV signals as well as SDTV signals. Accordingly, there is a need for a digital VTR that is capable of recording at two different data rates, e.g., at a first data rate required to record a HDTV signal and at a second data rate suitable for recording SDTV signals. Furthermore, it is desirable that such VTRs also be capable of supporting one or more long play modes of VTR operation.